The chance of floodwaters overtopping Manhattan’s seawalls and inundating parts of the city has skyrocketed at least 20-fold since the mid-1800s thanks to swelling seas and mounting storm surges finds a new study.

Researchers at Portland State University and Stevens Institute of Technology in Hoboken painstakingly analyzed tide gauge records back to the early-to-mid 19th century to draw startling conclusions about the Big Apple’s escalating flood risk (watch the video below, for details.)

They found the sea level has risen one and a half feet since the mid-19th century and that the rise in water or “storm tide” from 1-in-10 year storms has grown by almost a foot.

These two factors have elevated the height of what’s considered a 1-in-10 year flood in New York Harbor by about 2.4 feet over the last 170 years.

“What we are finding is that the 10-year storm tide of your great-, great-grandparents is not the same as the 10-year storm tide of today,” said Stefan Talke, the lead author of the study accepted for publication in Geophysical Research Letters.

A 10-year flood in the mid-1800s meant waters rose 5.6 feet; today such a flood raises the water 6.5 feet – and that’s before including the effect of the one and half foot rise in local sea level.

“The researchers found that today, waters can be expected to overtop the lower Manhattan seawall – 1.75 meters (5.74 feet) high — once every four to five years,” explains the study summary from the American Geophysical Union (AGU). “In the 19th century, when both sea levels and storm tides were lower, water was expected to overtop the Manhattan seawall only once every 100 to 400 years, according to the paper.”

In other words, the amped up surges and higher seas have “magnified the annual probability of overtopping the typical Manhattan seawall from less than 1% to about 20-25%” the study says.

Occurring “against a backdrop of increasing storm tides”, the authors found that Superstorm Sandy’s 11 foot storm tide in October 2012 was unrivaled in records dating back to 1821.

“Three of the nine highest recorded water levels in the New York Harbor (NYH) region have 10 occurred since 2010 (Mar. 2010, Aug. 2011, and Oct. 2012), and eight of the largest twenty have occurred since 1990,” the study’s abstract reports.

The reasons for the local sea level rise are well-understood: global warming has caused sea water to expand while melting glaciers and icesheets have deposited more water into the oceans. At the same time, the land surface has been sinking.

Explanations for the increase in the storm tide are more complex. Per the AGU study summary:

About half of long-term change could be attributed to decades-long variations in the North Atlantic Oscillation, an irregular fluctuation of atmospheric pressure over the North Atlantic Ocean that has a strong effect on winter weather in Europe, Greenland, northeastern North America, North Africa, and northern Asia.

Longer-term trends could also be influencing the increase in storm tides over the past two centuries, according to the paper. The authors speculate that climate change and increasing global temperatures could be contributing to the increase in storm tides. There could also be local factors, like deepening of shipping channels around New York harbor, that could have affected storm tides in the area over the past 170 years, Talke said.

The study underscores the vulnerability of New York City to coastal storms as sea level inexorably rises.

The Department of City Planning, in collaboration with other agencies, has undertaken a number of initiatives to build the city’s resilience. These studies are focused on identifying and implementing land use and zoning changes as well as other actions needed to support the short-term recovery and long-term vitality of communities affected by Hurricane Sandy and other areas at risk of coastal flooding.